Control theory deals with the behaviour of dynamical systems over time. The desired output of a system is called the reference variable. When one or more output variables of a system need to show a certain behaviour over time, a controller tries to manipulate the inputs of the system to realize this behaviour at the output of the system.
Take for example cruise control. In this case, the system is a car. The goal of the cruise control is to keep it at a constant velocity. So, the output variable of the system is the speed of the car. The primary means to control the velocity of the car, is the gas paddle.
A simplistic way to implement cruise control is to lock the position of the gas paddle the moment the driver engages cruise control. This is fine if the car is driving on perfectly flat terrain. On hilly terrain, the car will accelerate when going downhill; something its driver may find highly undesirable.
This type of controller is called an open-loop controller because there is no direct connection between the output of the system and its input. One of the main disadvantages of this type of controller is the sensitivity to the dynamics of the system under control.
To avoid the problems of the open-loop controller, control theory introduces feedback. The output of the system is fed back to the reference value. The controller then takes the difference between the reference and the output, the error, to change the inputs to the system under control.
Controllability is a measure for the ability to use a certain input to control an output of a system. In the cruise control example, an additional input to control the speed of the car is the clutch paddle. By varying the amount of power transferred from the engine to the wheels, you could control the speed of the car. However, most drivers will only use this method for very low speeds. The controllability of the car at high speeds is better with the gas paddle.